select cust_id from Orders where order_num in( select order_num from OrderItems where item_price>=10)

SELECT min(score) as min_score_over_avg FROM exam_record join examination_info using(exam_id) where tag='SQL' and score is not null and score>=(select avg(score) from exam_record join examination_info using(exam_id) where tag='SQL')

select difficult_level, sum(if(result='right',1,0))/count(*) as correct_rate from user_profile up join question_practice_detail qpd on qpd.device_id=up.device_id join question_detail qd on qd.question_id=qpd.question_id where university='浙江大学' group by qd.difficult_level ORDER BY correct_rate;

SELECT day(date) as day,count(*) from question_practice_detail where month(date)=8 group by day